1. Field of the Invention
This invention relates to forced air heaters and in particular to indirect-fired air heaters for portable personnel shelters and the like.
2. Description of the Prior Art
The prior art includes a variety of indirect-fired air heaters suitable for heating portable personnel shelters, vehicle interiors and related applications.
As shown in U.S. Pat. No. 2,421,370, indirect-fired air heaters generally include an outer casing, a smaller drum enclosing a combustion chamber positioned within the casing to define a plurality of air heating passages extending through the heater between the periphery of the drum and the casing, and a fan secured at one end of the casing suitable for forcing venting air through the heating passages. As shown in the foregoing patent, a fuel burner is positioned at the upstream end of the drum to accommodate the delivery of an axially directed flame into the combustion chamber toward its downstream end whereat the resulting combustion gases are in turn evacuated from the chamber.
When this type of burner is put into operation, the peripheral region of the combustion chamber surrounding the burner is relatively cool compared to its downstream end where the temperature within the chamber is generally in excess of 1100.degree. C. Due to this temperature differential along the length of the combustion drum, only 60-70 percent of its surface area is believed to be effectively utilized to heat the air moving through the casing. Thus, in order to obtain sufficient heat transfer from the drum to the venting air moving through the air heating passages, it has been the practice to provide a series of vanes or baffles at the downstream end of the drum to increase the radiant surface area exposed to the venting air flow about the periphery of the drum, or alternatively, provide a relatively large number of smaller heating passages as in the arrangement shown in the above cited patent. As will be appreciated, both of these approaches result in a substantial pressure drop in the air flowing through the heater as well as being relatively difficult to fabricate and repair. Moreover, while the foregoing arrangements have been for the most part satisfactory when used with lighter fuels such as gasoline, when heavier, slower burning fuels such as diesel oil are used to fire the burner, it is difficult to obtain complete combustion of the fuel. As a result, carbon tends to build up on the walls of the combustion chamber. This in turn leads to corrosion and pitting in the walls which ultimately renders the drum unserviceable requiring its replacement.
In contrast to the foregoing arrangement, U.S. Pat. No. 3,312,212 discloses an indirect-fired office or warehouse heater wherein the combustion gases are evacuated from the combustion chamber at the upstream or burner end of the chamber. As shown in that patent, the combustion gases flow from the chamber through a plurality of tubes connected with a flue gas header which is aligned within the heater downstream from the combustion drum. While this arrangement utilizes a greater percentage of the drum's surface area to heat the venting air flowing through the heater, the necessarily square or rectangular cross-sectional configuration of its combustion drum is relatively bulky, and, without additional reinforcement, less explosion resistant than heaters having a generally cylindrical combustion drum such as that shown in U.S. Pat. No. 2,421,370 noted above.